Cryotherapy or, the application of non-invasive cold modalities, results in a decrease in tissue temperature and is widely used in sports and rehabilitation to aid recovery and injury management (Costello et al., 2012). Cooling decreases blood flow (Thorsson et al., 1985), nerve conduction velocity (Algafly and George, 2007; Herrera et al., 2010), and muscle spindle activity (Oksa et al., 2000). When cooling is applied prior to physical performance, these physiological changes and interactions between those various systems may eventually lead to a deterioration in physical performance (Bleakley et al., 2012; Drinkwater, 2008).

Knicker and colleagues (2011) described that physical performance can be assessed at muscle, exercise or competition level. Muscle performance tests aim to determine the complete functioning of a single muscle through repeated contraction (e.g. maximal voluntary contraction) and are often measured by electromyography (EMG). In contrast, functional performance tests cover whole-body exercise tests that are easy to use in laboratory or field settings (i.e. repeated sprints, hop and jump tests, agility tests) and can be assessed in either a quantitative or qualitative way (Knicker et al., 2011).

In terms of muscle performance tests, a linear relationship between increasing force and, EMG amplitude and total power of the EMG power spectrum can be observed (Drinkwater, 2008). The application of different cooling modalities to muscle tissue induces changes to the neuromuscular activation pattern and could eventually lead to an unfavorable effect on muscular performance (Vieira et al., 2013). Drinkwater (2008) reported an increase of the EMG signal during various contraction types as long as the intra-muscular temperature remains above 20[degrees]C. These findings can also be translated to changes in mechanical muscle properties, namely an increase in stiffness and tension, and a decrease in elasticity of the muscle (Mustalampi et al., 2012). However, when intra-muscular temperature decreases below the 20[degrees]C threshold, a substantial reduced EMG amplitude can be observed, which could lead to a decrease in muscle strength (Drinkwater, 2008).

Bleakley et al. (2012) performed a systematic review on the effect of local pre-cooling on functional performance, and found that muscle strength, vertical jump, sprint and agility performance immediately deteriorated after cooling, but also concluded that there is still a lot of conflicting evidence on the effect of cryotherapy on functional performance outcomes. Moreover, most studies examining functional performance deficits used aggressive or short-term cooling methods (e.g. cold packs, cold water immersion, ice bags) and were not able to standardize the administered treatment temperature accurately (Bergh and Ekblom, 1979; Cross et al., 1996; Evans et al., 1995; Fischer et al., 2009; Patterson et al., 2008; Richendollar et al., 2006).

In the last decade computer controlled cooling devices started emerging in both orthopedics, rehabilitation and sports. In contrast to previous, primitive cooling modalities (e.g. ice bags, cool packs), computer controlled cooling devices have the ability to cool and control the administered temperature very precisely and distribute it equally across the joint or muscles. Recently, Hohenauer et al. (2017) used a continuous computer controlled cooling application to examine the effect on maximal and submaximal voluntary contractions related to peripheral and central fatigue. However, they found no changes in isometric maximal voluntary contraction (MVC) of the quadriceps after a 20 minute cooling period at a temperature of 8[degrees]C (Hohenauer et al., 2017). Nevertheless, these cooling devices are more time dependent than conventional cooling methods and therefore need more time to achieve significant temperature decreases in subcutaneous tissues (e.g. muscle). This could clarify why no differences in MVC were found after cooling for only 20 minutes. However, the potential perturbations that cooling can cause at muscle level and its relation to fatigue could give us new insights regarding how muscle and functional performance interact. To our knowledge, the use of a computer controlled continuous cooling application has never been used to study its effects on neuromuscular activity and functional performance.

Therefore, the purpose of this study was to examine if a low temperature computer controlled continuous knee cooling protocol (10[degrees]C) for one hour and a moderate temperature computer controlled continuous knee cooling protocol (18[degrees]C) for one hour affected neuromuscular activity of the musculus quadriceps vastus medialis and functional performance tests..

Methods

Trial design

We used a randomized controlled study design. Independent variables were the study population, the cooling intervention and the time between pre- and post-tests. The primary dependent variables were distance for the Single Leg Hop for Distance Test (SLHD), time for the Six Meter Single Leg Crossover Hop Test for Time (COHT), and EMG activity during MVC of the quadriceps vastus medialis. A secondary dependent variable was skin temperature of the knee.

Participants

Twenty healthy male subjects (age = 24 [+ or -] 3 years, length = 1.82 [+ or -] 0.07 m, weight = 76.05 [+ or -] 9.53 kg) with no history of injury or surgery to the lower extremity were included in this semi-crossover study and randomized in either Group 1 (10[degrees]C-protocol versus no cooling) or Group 2 (18[degrees]C-protocol versus no cooling) by using a random sequence generator and were blinded for the intervention temperature. Participants always received the same cooling protocol temperature, once they were allocated to a designated group. The cooled leg always served as the intervention condition and the non-cooled leg as the time-matched control condition within the designated group in order to be sure that we were measuring the effect of cooling and not the effect of sitting down for one hour (see Figure 1). Demographic characteristics of the participants are shown in Table 1. Participants were excluded when they sustained an injury during the trial period, had a BMI [greater than or equal to] 30, or stated contraindications to cryotherapy (e.g. cold allergy). Subjects were physically active, practicing sports minimal two times a week and had a normal BMI (BMI = 18.5 - 25).

The study was approved by the institutional medical ethics committee of the university hospital UZ Brussels and Vrije Universiteit Brussel (Belgium) (B.U.N. 143201629149). All participants were provided written and oral information about the experimental procedures and possible risks, and signed the written informed consent before the experiment. All procedures were conducted according to the Declaration of Helsinki.

Procedures

Participants came to the laboratory twice: on day one, subject characteristics as age, length, and weight were registered. Next, participants performed a MVC of the quadriceps vastus medialis, SLHD, and COHT with both legs before and after the cooling of their right leg. At day two, the same tests were performed with both legs before and after cooling of the left leg. All performance tests were always executed with the cooled leg first, to maximize the effect of cooling, then with the non-cooled leg. On both test days, subjects were scheduled at the same time of day. Mean ambient temperature during these trials was 26.3 [+ or -] 2.1[degrees]C.